1. Field of the Invention
The present invention relates to a control device of an electric motor to be rotated at a comparatively low speed which is to be used in a disk device such as a CD or a DVD, and more particularly to a control device of an electric motor which accurately detects a rotating position and the number of rotations and carries out braking from a rotating state to a stop state promptly and accurately.
2. Description of the Related Art
Conventionally, as the methods known for stopping a three-phase motor having a driving winding for each phase being provided in a stator in a rotation state, there are so-called a short braking method where a full driving winding is short-circuited, and also known is so-called a reverse rotation braking method where a conduction state to each phase driving winding of an electric motor is made in a different state from a normal driving state such as to subject a driving winding in a conduction state under a period so as to generate a back electromotive force by which more than a source voltage can be applied for carrying out the braking.
More specifically, as for a method used for braking an electric motor having a comparatively low rotating speed, it is the reverse rotation braking method that is used because it is capable of carrying out an effective braking from a normal rotation state to a stop state.
However, since a high-speed rotation of a disk (for example, 10,000 RPM) has recently been introduced into the market, reflecting a demand for the high-speed write/read of data into/from the disk, the applicability of the reverse rotation braking method comes to b limited for its use. This is because in the case in which the reverse rotation braking method is used for braking the electric motor to carry out the high-speed rotation, the amplitude of the back electromotive voltage of the driving winding is necessarily increased to be more than a source voltage. As a result, the insulation intensity of the electric motor is required to be increased or a power loss problem which leads to heat generation cannot be any more disregarded.
In order to resolve such a situation, Japanese unexamined patent publication JP hei 10-98894 discloses such a method that the rotating speed signal of the three-phase motor to be unipolar-driven is compared with a predetermined rotation number threshold, in case of so-called a high-speed rotation area in which the rotating speed signal is higher than the threshold signal, one of the ends of a full driving winding is grounded to apply a short braking method, while in case of being in so-called a low-speed rotation area in which the rotating speed signal is smaller than the threshold value, a braking method is switched from a short brake to a reverse rotation brake. In this way, an electric motor to carry out a high-speed rotation can be braked.
Moreover, JP hei 8-275571 discloses the driving operation of a single-phase motor to be stopped which switches a conducting direction to a driving winding to carry out a rotation in response to the position detecting signal of a rotating sensor (a Hall element). In this case, the conducting direction is switched to carry out a reverse rotation, meanwhile, if the rotating sensor detects motor speed in that a predetermined number of rotations is obtained, then the switching operation is stopped, and the conducting direction to the driving winding is fixed to one direction whereby the stop operation can be carried out.
While switching to a reverse rotation brake is carried out in a low-speed rotation area in the conventional Patent Document 1, however, the reverse rotation braking method controls a phase switching timing by detecting the rotating position of the electric motor. More specifically, a sensorless motor detects a rotating position thereof according to a change in the polarity of an induced voltage where the phase is switched in the detected timing. However, when the number of rotations of the electric motor is decreased, the induced voltage is reduced. Consequently, the phase switching timing might be likely to be detected erroneously. Thus, there is a possibility that the switching might be carried out in an erroneous timing. Moreover, there is also a problem in that the rotation is carried out in a reverse direction by an inertia so that unpredictable induced voltage is further detected by a reverse rotation that is being kept by said inertia. These problems make the electric motor out of a control. This situation also brings about such a practical problem that replacing a disk in an audio apparatus immediately after a stop operation might damage the operator's fingers and it is not visually good to see the disk still continuing its turning. Further, this sometimes brings about a problem for the need of replaying being carried out immediately thereafter.
In said JP hei 10-98894, moreover, the middle potential of a Y connection fluctuates in case of bipolar driving in which a power is supplied to two of driving windings having a three-phase Y connection at the same time in positive and negative directions. Therefore, the induced voltage having each phase cannot be detected accurately. For this reason, the method in the Document 1 cannot be applied.
In the conventional method according to said JP hei 8-275571, moreover, the conducting direction to the driving winding is fixed to one direction when a brake is applied to a single-phase motor. In addition, there is also a problem in that a Hall device for detecting a rotating position is requited.